1. Field of the Invention
The present invention relates to a method for the continuous production of nixtamalized corn flour and, more particularly, to one that achieves precooking of the whole corn with a lime solution, and the use of an adiabatic cooling and classifying under a partial vacuum during the manufacture of whole nixtamalized corn flour for the preparation of tortilla and whole grain corn-based foods.
2. Description of Related Art
Nixtamalized corn flour (NCF) is conventionally produced by alkaline cooking of food-grade corn under atmospheric pressure, steeping and washing, grinding the partially cooked corn (nixtamal) and drying to give corn masa flour. This flour is sieved and blended for different food product applications and it is usually supplemented with food-grade additives before packaging for commercial table tortilla and snack production. Maseca® is the main brand flour in the US and Latin America followed by Minsa, Ill. Cereal Mills (Cargill) and Quaker Oats. Bressani (Sustain, 1997) described both batch and continuous processes for cooking corn to produce instant masa flour. In the modern continuous process, the lime used is from 0.6 to 1.2% (based on corn) and mixed with equal parts of corn and potable water. The corn suspension is cooked by atmospheric steam injection and although the pericarp is partially removed during cooking and washing, there is still some fiber left from the corn (with a 7%-9% solid loss).
The traditional method of cooking maize with lime (1-2%) at the rural level is time consuming (around 14 to 15 hours) and requires hard labor. From 70% to 80% of the time is taken by the cooking and steeping-washing operations (a 1:3 corn to water ratio); that time can be decreased to 8-12 hours or less at the urban or cottage level (with a 11 to 13% corn loss; Bressani, 1990 in Sustain).
At the industrial or commercial level, the “grinding and dehydration” steps are major cost factors. In any case, the availability of an instant corn flour has many advantages such as convenience for the customer, less labor, lower water and energy use, a safe and stable product (3 to 4 months under normal storage conditions), and the possibility to use the flour as a carrier of a macronutrient (soybean, wheat and amaranth), micronutrients (amino acids, vitamins and minerals) along with novel nutraceuticals (prebiotics).
Corn and tortilla chips are the second most popular salted snack food in the USA, after potato chips. Any method which may decrease both time and cost, and still yield an acceptable nixtamalized corn dough or masa flour product per kilogram of raw corn, would be advantageous to the final customer. The mature dent corn kernel (Watson, 2003; 1993 in fao.org) has five separable components, on a dry weight basis: tip cap (0.8-1.1%), pericarp/hull (5.1-5.7%) and aleurone (2%), endospenn (81.1-83.5%) and germ (10.2-11.9%). The pericarp contains 90% insoluble fiber (67% hemicellulose-heteroxylans-, 23% cellulose and 0.1% lignin). Although bran is sometimes used as a synonym for pericarp, in the dry or wet-milling process the bran includes the pericarp, tip-cap, aleurone layer-isolated with bran- and adhering pieces of starchy endosperm.
There have been a few studies on indusrial corn arabinoxylans (soluble bran: Wolf et al. 1955) and lime-treated Maseca® pericarp and bran components (Vidal et al. 1999; MX Patent 206738). The food-guide pyramid (2005 in Mypyramid.gov) suggests eating half of your grains whole (6 oz. or grain-servings/day) along with 4.5 cups of fruit-vegetables/day for a 2000 calorie-diet. Only 8% of the U.S. population is estimated to consume at least 3 servings of whole-grain and 3 cups of fruit-vegetables/day. Modern eating habits are incresing both obesity and metabolic syndrome (47 million U.S. residents) with increased risk for cardiovascular disease and type-2 diabetes (AHRQ.gov). Starch is more digestible and its glycemic response increased 2-3 fold compared with coarsely ground flour or whole grain (Colagiuri et al. 2002). Therefore, a “calorie restriction” (30-40%) in high-fat/protein and sugar/starchy foods will also reduce the daily intake to maintain a healthy body-weight.
A whole-grain corn definition by the FDA has been requested (AACC, 2005) such that a whole-grain Nixtamalized corn masa flour (WNCF) has a 7.3% to 9.6% dietary fiber content. The AACC International (2006) comments on part II of the FDA's Draft Guidance on Whole Grain Label Statement provide: Cereals and Pseudocereals that, when consumed in whole form (including the bran, germ and endosperm) are considered whole grains. If intake is unchanged (Marquat, 2006), formulation modifications by adding whole-grain flours to existing cereal-based products would increase (50%) the number of 16-g whole-grain servings/day from 2.2 to 3.3. Both USDA and FDA have said that 51% of the total ingredients must be whole grain.
In this connection, reference is made to the following U.S. Pat. Nos. 6,387,437, 5,447,742, 4,594,260 and 4,513,018. These prior art methods for the industrial production of nixtamalized corn flour and dry masa flour involve high-temperature cooking and dehydration with low energy requirements for a high yield of the end product.
Two transport processes occur simultaneously during a cooling operation: a) heat transfer from convection, conduction and evaporation of free water, and b) mass transfer due to partial vapor-pressure gradients and due to total pressure gradients caused effected by high temperature or external-pressure. Total pressure-driven mass flow within a food may not only occur in high-temperature convective-transfer but also in moderate-temperature vacuum-transfer (Crapiste et al. 1997 and Rotstein, 1990). Phenomenological laws such as Newton's, Fourier's, Fick's and Darcy's law are based on proportionalities between a flux and a driving force. When more than one driving force is present in a coupled heat conduction and mass diffusion (moisture-transfer potential), cross-effects can occur such as the Soret (gradient temperature can cause mass transfer) and Dufour effect (gradient concentration can effect heat flow) in hygroscopic capillary-porous foods (Okos et al. 1992).
Having in mind the disadvantages of the prior methods, several applications were conducted to improve the efficiency of cooling by means of convective cooling (ie., using continuous heat-exchanger and dynamic/static equipment) as in U.S. Pat. No. 7,220,443 and Japanese patents (JP2002-309266, JP09-194848, JP54-50479 and JP53-125908). Other recent innovations related to adiabatic cooling (ie., using batch equipment with vacuum pumps and steam jets) as reflected by the following U.S. Pat. Nos. 6,025,011, 5,401,522 and 4,331,690. In a reduced-pressure cooling, the temperature falls due to the latent heat until an equilibrium temperature is reached where the heat absorbed from the environment is equal to the heat loss during vaporization at the food surfaces or within pores. With the use of vacuum processing, the residence time of the food optimal for contaminating microbes is minimized, thus achieving a better sanitation/shelf-life than slower processes.
Classification is the separation of a particulate material into a coarse and fine fraction. The segregation has always been by size in flour production by using a mesh that allows only particles of a predetermined maximum size to pass through screens/sieves (from a large to a medium particle size range of 500 mm to 100μ or U.S. standard mesh 100). Other non-conventional methods have used density which is a major factor in centrifugal separators (cyclones) where fluid drag and gravitational forces are involved (Zenz, 1983 and Klumpar et al. 1986). Depending on the equipment used, dry classification might also be affected by size shape/distribution, electric, magnetic and surface properties. Air classifiers (static and dynamic ones) are widely used in the food industry to remove a solid from a clean gas generated by internal/external fans (with a low-pressure drop). In such equipment, classification is effected in a stream of air from a medium down to a submicron particle-size range (1000μ: U.S. mesh 18 down to 40μ or <U.S. mesh 400 with >99.9% efficiency). A key feature of a vortex-type separation is that the outer area moves slowly and the center moves fast. As the fine particles are moved inward they are also sucked into the inner vortex, frictional resistance is reduced and the particle speed is increased (similar to Coriolus effect due to earth's rotation).
Although the above described prior art methods can be useful useful in producing an increased yield of nixtamalized corn flour with a low-cost industrial application as well, a continuous process was still unavailable for the production of whole nixtamalized corn flour not only using a whole-grain corn precooking but also involving an adiabatic cooling with airflow classification under vacuum was still unavailable in the market at the time of the invention.